1. Field of the Invention
This invention relates to a process for producing a highly bulky nonwoven fabric by the use of heat-adhesive composite fibers having three-dimensional apparent crimps and substantially no latent crimpability.
2. Description of the Prior Art
Porous nonwoven fabric obtained by using heat-adhesive composite fibers whose composite components are fiber-forming polymers of different melting points have been known (Japanese patent publication Nos. Sho 42-21318/1967, Sho 44-22547/1969, Sho 52-12830/1977, etc.). Crimps which are developed when composite fibers are stretched and then relaxed (such crimps will hereinafter be often referred to as apparent crimps), are spiral, three-dimensional crimps. Apparent crimps are known to impart bulkiness to the fibers, and have been utilized in the fields of wadding for counterpane, etc.
However, heat-adhesive composite fibers consisting of polymer components of different melting points and having apparent crimps have drawbacks. For example when the fibers are subjected to heat treatment for heat-adhesion, additional crimps generally develop (such crimps being brought about by "latent crimpability" of the fibers), resulting in a large shrinkage of the fibers; hence homogeneous nonwoven fabric cannot be obtained and the bulk of the resulting web is reduced as compared with that prior to heat treatment.
To avoide such a shrinkage due to latent crimpability generated at the time of heat treatment when making a nonwoven fabric from such fibers, a process has been proposed wherein composite fibers are annealed in advance of making a nonwoven fabric from the fibers to thereby make the latent crimp-ability apparent in advance. According to the process, however, it is difficult to control the number of crimps. If the number of crimps becomes too large, interfilamentary entanglements become too firm at the time of web formation and reduce the bulk of the web. To the contrary, if the number of crimps is too small, an obstacle occurs at the time of processing the fibers into a web in that interfilamentary entanglements are insufficient and thereby reduce the bulk of the web.
Thus it is the present status that porous nonwoven fabrics comprising heat-adhesive composite fibers according to the prior art have not been used for substantial application in fields needing bulkiness, such as wadding for kilts.
The present inventors have made strenuous studies for obtaining a highly bulky nonwoven fabric without the above-mentioned drawbacks and as a result have attained the present invention.